Such belt retractor has a frame, a belt reel which is rotatably mounted in the frame, a locking mechanism which is adapted to block a rotation of the belt reel in an unwinding direction of the safety belt vehicle-sensitively and/or sensitively to the belt webbing, and a pretensioning drive which is adapted to drive the belt reel in a wind up direction of the safety belt and in the unwinding direction.
The belt retractor is part of an active vehicle occupant restraint system by means of which, upon recognizing a critical vehicle situation, various precautions are taken to protect a vehicle occupant from negative consequences in the best possible way. The measures taken may, for example, include changing the inclination of a back rest of a vehicle seat, closing the vehicle windows, pretensioning the safety belt, and further measures. All these measures are reversible; as soon as the vehicle state that was recognized as being critical does no longer exist, the vehicle is again returned to its initial state. As regards the belt retractor, this means that the safety belt that was wound up by the pretensioning drive upon recognizing a critical vehicle situation to eliminate, as far as possible, the so-called belt slack in the safety belt, is again released, as soon as the vehicle state recognized as being critical does no longer exist. The possibility to actively release the safety belt again and re-establish the original state clearly distinguishes a pretensioning drive from a conventional belt tensioner drive. The latter solely serves to tension the safety belt directly before an accident; no return feature is provided. A further distinction between a conventional belt tensioner drive and a pretensioning drive is the belt webbing force achieved when the safety belt is tensioned. The belt webbing forces generated by a pretensioning drive are several times smaller than those generated by a conventional belt tensioner drive (up to clearly more than 1000 N).
In belt retractors having a pretensioning drive, the locking mechanism may be activated “by accident”, when the belt reel was rotated in the wind up direction of the safety belt and the safety belt was pretensioned, for example if the vehicle is braked. This may result in that a locking pawl triggered by the locking mechanism completely engages in a locking toothing associated therewith, so that the belt reel is completely blocked. Since the locking toothing normally is undercut, i.e. when being subjected to a load it exerts a force entraining the locking pawl into the locking toothing so as to be able to reliably transfer the high locking loads acting in an accident between the locking toothing and the locking pawl, a reverse rotation of the belt reel in the wind up direction is necessary to permit the locking pawl to be moved out of the locking toothing again. It follows from this that prior to releasing the pretensioned safety belt, the belt webbing force at first increases because of the rotation of the belt reel to release the locking pawl. Since the safety belt has already been pretensioned anyway, the vehicle occupant experiences this as being unpleasant.
It is, therefore, the object of the invention to further develop a belt retractor of the aforementioned kind to the effect that the increase of the belt webbing force when returning from a state in which the safety belt is pretensioned to the initial state may be prevented.